In many applications, amplification of a weak electrical signal is desired and necessary. For example, in high-speed serial receivers, a signal received over a communication link may be comparatively weaker than the noise contributed by processing stages in the receiver. Therefore, prior to substantive processing, the weak signal is often amplified such that the received signal is not “lost” within the noise.
In general, amplification of a signal may be performed by a fixed or variable gain amplifier (VGA). More complex designs, such as high-speed serial receivers, may make use of a VGA within an automated gain control (AGC) loop. In an AGC loop, the output of the VGA is feed back to a controller, which adjusts the gain of the VGA based on a function of the VGA output.
The performance of an AGC loop typically depends on several factors, including gain linearity of the VGA and variation in the VGA bandwidth over the range of available gain settings. A resistively degenerated amplifier is one desirable architecture for the implementation of a VGA within an AGC loop. A resistively degenerated amplifier exhibits excellent gain linearity and immunity to process variations. However, the conventional architecture for a resistively degenerated VGA suffers from bandwidth variation over the range of available gain settings.
Therefore, there exists a need for a resistively degenerated VGA that provides for constant-bandwidth over the operating gain range.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.